In Kolbe's reaction, instead of phenol, phenoxide ion is treated with carbon dioxide. Why?
In phenoxide ion, the ability to give a lone pair of electrons to the benzene ring is more in comparison to phenols. Therefore. the reactivity of phenoxide ion towards electrophilic substitution reaction is more in comparison to phenols.
Thus, phenoxide ion being a stronger nucleophile reacts easily with CO2, (weak electrophile) than phenols in Kolbe's reaction.
Note Kobe's process is also known as Kolbe - Schmitt reaction. This reaction is a precursor to aspirin.